(1) Field of the Invention
The present invention relates to a magneto-optical head device, and more particularly to a magneto-optical head device for recording, reproducing and erasing data on a magneto-optical disk.
(2) Description of the Related Art
In recent years, for purposes of size and cost reduction of the magneto-optical head device, research is being made for development of a structure utilizing a diffractive element whose diffraction efficiency is dependent on the direction of polarization of incident light.
An example of a conventional magneto-optical head device using such diffractive element as above is shown in FIG. 1 (description of which is found, for example, in Japanese Patent Application Kokai Publication No. Hei 4-232633). In such conventional magneto-optical head devices, the light emitted from a semiconductor laser 20 is transmitted through a polarizing diffraction grating 15 whose diffraction efficiency is dependent on a polarization direction of incident light and, after being converted to parallel light by a collimator lens 14, is converged on a magneto-optical disk 12 by an objective lens 13. The reflected light from a magneto-optical disk 12 is again converted to parallel light by the objective lens 13 and, after being transmitted through the collimator lens 14, is diffracted by the polarizing diffraction grating 15. The +1st order diffracted light and the -1st order diffracted light from the polarizing diffraction grating 15 are received respectively by photodetectors 18 and 19 equipped with analyzing function. The analyzing-function-equipped photodetectors 18 and 19 are constructed such that analyzers 16 and 17, which transmit only a predetermined polarized light component of the incident light, are bonded respectively on photodetectors 21. The analyzers 16 and 17 are disposed so as to allow the polarization directions of the transmitted light to be orthogonal to each other. Thus, the magneto-optical signal is detected from a difference between the outputs of the analyzing-function-equipped photodetectors 18 and 19. The photodetectors 21 used in the analyzing-function equipped photodetectors 18 and 19 are divided respectively into three light receiving sections 21a-21c and 21d-21f. Appropriate calculation of outputs from the light receiving sections enables detecting a focusing error signal and a tracking error signal.
The polarizing diffraction grating 15 in the conventional magneto-optical head device described above has characteristics wherein the diffraction efficiency is dependent on the polarization directions of the incident light. Specifically, it is designed such that the 1st order diffraction efficiency of the polarized light component (P-polarized light component) parallel to and of the polarized light component (S-polarized light component) perpendicular to the grooves in the grating are respectively 30% and 5%. Generally, where the width of a line portion and that of a space portion of! the diffraction grating are the same and the phase difference is assumed .alpha., the transmittivity and the 1st order diffraction efficiency are given respectively by cos.sup.2 (.alpha./2), (2/.pi.).sup.2 sin.sup.2 (.alpha./2). From this, the transmittivities of the P-polarized light component and the S-polarized light component in the conventional example may be calculated respectively as 26% and 87.7%.
The polarization direction of the light emitted from the semiconductor laser 20 makes an angle of 45.degree. with respect to the P polarization direction. Therefore, assuming that the polarization direction of the light transmitted through the polarizing diffraction grating 15 and then incident on the magneto-optical disk 12 makes an angle of .theta. with respect to the P polarization direction, the angle will be: EQU .theta.=tan.sup.-1 (0.877/0.26).sup.1/2 =61.4.degree.
The component (main polarized light component) in the polarization direction in the average of the reflected light from the magneto-optical disk 12 is the same as the incident light, and the magneto-optical signal component due to Kerr effect is orthogonal thereto.
The ratio of the magneto-optical signal component diffracted by the polarizing diffraction grating 15 and then led to the analyzing-function-equipped photodetectors 18 and 19 will be: EQU 0.3.times.2.times.sin.sup.2 .theta.+0.05.times.2.times.cos.sup.2 .theta.=0.486
The analyzers 16 and 17 transmit only the P-polarized light component and the S-polarized light component, respectively, so that the ratio of the magneto-optical signal component reaching the two photodetectors 21 will be a half, that is, 0.243. In order to obtain a good signal-to-noise ratio during the reproduction operation, it is necessary to raise the ratio of the magneto-optical signal component that reaches the photodetectors used for detecting magneto-optical signals. In the prior art, this ratio is 0.243, which is low, so that there is a deterioration in the signal-to-noise ratio.
On the other hand, assuming that the main polarized light component diffracted by the polarizing diffraction grating 15 and led to the analyzing-function-equipped photodetectors 18 and 19 makes an angle of .phi. with respect to the P polarization direction, the angle .phi. will be: ##EQU1## Since the analyzers 16 and 17 transmit the P-polarized light component and the S-polarized light component respectively, the amounts of the main polarized light component beams which reach the two photodetectors 21 are not the same as each other. In order to obtain a good signal-to-noise ratio during the reproduction operation, it is necessary to equalize the amounts of the main polarized component beams which reach the two photodetectors used for detecting the magneto-optical signals and to raise a common-mode noise elimination ratio in the differential detection process. In the exemplified prior art, such amounts are not equal with each other so that the signal-to-noise ratio is deteriorated.
Thus, although the conventional magneto-optical head device is suited to make the device compact and low cost, there is a problem that no satisfactory signal-to-noise ratio is obtained during the reproduction operation.